backup failed

I'm using Acronis 2011, I am backing up onto a internal hard drive, and a external hard drive. I'm also using the differential method, and I have the settings set to delete the old backups to make room for the new backups. But when ever I check up on the status of my backup, I get the error message "backup failed"
here is the reason why the backup failed but it's all Greek to me, does anyone know what this means.
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